The term "swept signal analysis instrument" as used herein is a generic term referring to any electronic equipment in which an input signal is mixed to an intermediate frequency using a swept local oscillator and is subsequently filtered.
The most common swept signal analysis instruments are spectrum analyzers and network analyzers, and it is with reference to these instruments that the present invention is illustrated. In a spectrum analyzer, an input signal to be analyzed is heterodyned to an intermediate frequency (IF) using a swept local oscillator. The IF signal is filtered with a narrow bandwidth IF filter. The swept local oscillator has the effect of "sweeping" all the frequencies of the heterodyned-down input signal past the fixed frequency of the IF filter, thereby permitting the filter to resolve the input signal's spectral composition. The signal power within the filter bandwidth is determined by a detector cascaded after the IF filter and is typically displayed on a graphical display associated with the instrument.
A network analyzer is similar in many respects to a spectrum analyzer but instead of analyzing an unknown signal, the instrument analyzes an unknown network. To do this, the instrument excites the unknown network with a known signal and monitors the phase and amplitude characteristics of a resultant signal, thereby permitting the network's transfer function to be characterized. Again, the instrument relies on a swept local oscillator to heterodyne the input signal to an intermediate frequency, and the IF signal is again filtered prior to analysis. In a network analyzer, however, the IF filter serves to eliminate noise effects rather than to provide a narrow resolution bandwidth, and the filtered IF signal is analyzed to determine the phase and amplitude of the IF signal rather than its power. The analysis further includes "normalizing" the IF signal to the original excitation signal in order to reduce excitation source related errors.
All swept signal analysis instruments suffer from a common limitation, namely measurement errors caused by the sweeping operation. In spectrum analyzers, these errors manifest themselves as a degradation in the performance of the IF filter. As the sweep speed increases, spectral components of the input signal are swept at increased speeds through the filter. The behavior of the filter to these quasi-transient signals can be optimized by using a Gaussian filter response, thereby minimizing conventional dynamic problems such as ringing and overshoot. However, above a certain sweep rate, even an ideal Gaussian filter becomes unsatisfactory due to spreading of the filter passband and errors in amplitude (i.e. power) response. In particular, the filter passband approaches its impulse response shape when the sweep rate increases to infinity, and the amplitude of the response decreases with the square root of the sweep rate.
An ideal Gaussian response is often approximated by cascading a plurality of single-tuned filter stages. (A true Gaussian response cannot be physically realized since it is noncausal.) These cascaded stages generally include capacitors, inductors or crystals, and thus have transfer functions with poles. All transfer functions with poles exhibit non-flat group delay, also known as nonlinear phase response. One problem with such filters is that they respond more quickly to the leading edge of an input transient signal than the trailing edge. Another problem is that the trailing edge often exhibits notches and ringing rather than a smooth fall to the noise floor. Small signals are difficult to differentiate from aberrations on the falling edge of nearby larger signals.
These problems, in conjunction with the magnitude errors caused by fast sweeping, have limited traditional analyzers to a maximum sweep rate of one half the filter bandwidth squared (0.5 BW.sup.2). At this rate, the magnitude error is about 1.18%, or less than 0.1 dB--generally considered to be an acceptable accuracy.
The mathematical derivation of the amplitude error and of the passband distortion resulting from fast sweeping in known in the art, as shown by Appendix B of Hewlett-Packard Application Note 63, May, 1965, and by Tsakiris, "Resolution of a Spectrum Analyzer Under Dynamic Operating Conditions," Rev. Sci. Instrum., Vol. 48, No. 11, November, 1977. These references are available, inter alia, from the Patent Office file on U.S. Pat. No. 5,117,179, Ser. No. 07/493,590, which is a public record.
In network analyzers, sweep related errors manifest themselves as errors in normalization and as irregularities in both the frequency and phase response of the noise limiting filter. Again, these errors increase with sweep speed and limit the maximum rate at which a network analyzer can sweep through a frequency range of interest.
In accordance with the present invention, the sweep rate limitations that heretofore have constrained the maximum sweep rates of swept analysis instruments are obviated by optimizing the filter circuitry and post-processing the IF signal using various techniques to compensate for fast sweeping errors.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description thereof, which proceeds with reference to the accompanying drawings.